Ferroelectric phase shifters are typically used to phase shift radio frequency signals for use in, for example, steering microwave signals in electronic scanning arrays. Commonly assigned U.S. Pat. No. 5,212,463, the entire disclosure of which is incorporated herein by reference in its entirety, discloses a planar ferroelectric phase shifter in accordance with the present invention. The phase shifter disclosed in U.S. Pat. No. 5,212,463 is an inexpensive, easily manufacturable alternative to ferrite phase shifters for steering microwave radar beams and is compatible with commonly-used microwave transmission media.
In general, the ferroelectric phase shifter is a microstrip circuit having a ferroelectric material interposed between a conductor line and a ground plane. The conductor line typically includes an impedance transformer for matching the impedances at the material interface between the nonferroelectric and ferroelectric materials. In this manner, the impedance transformer reduces signal reflections. A microwave signal input to the phase shifter emerges from the transformer and travels through the ferroelectric material between the conductor line and the ground plane.
As is known in the art, the dielectric constant of the ferroelectric material affects the speed of a microwave signal propagating through the phase shifter and, thus, causes a phase shift. The dielectric constant of the ferroelectric material, however, can be varied by a DC voltage applied across the ferroelectric between the conductor line and the ground plane. Typically, DC voltage is supplied by an outside DC power supply through a high-impedance, low pass filter preventing microwave energy from entering the DC supply. An inductive coil or other appropriate circuit may serve this role. A DC blocking circuit is typically used to confine DC voltage to a select region of interest only in the microstrip circuit. Conventional DC blocking circuits include coupled lines and chip capacitors. U.S. Pat. No. 5,212,463, as well as the article "High Voltage DC Block for Microstrip Ground Planes", Electronics Letters, Aug. 2, 1990, Vol. 26, No. 16., by Thomas Koscica, disclose high-voltage DC bias blocking circuits in the ground plane of a microstrip circuit.
In practice, the dielectric constant of a ferroelectric material will change over time due to several factors including temperature, humidity, aging of the material and hysteresis. Therefore, it is important to know the dielectric constant of a phase shifter's ferroelectric material during operation to ensure effective operation of the phase shifter.
A disadvantage associated with presently available ferroelectric phase shifters is that they can only be calibrated during the fabrication process. The degree to which temperature, humidity, aging, hysteresis and the like cause the value of the dielectric constant to vary over time depends on the particular material. Ferroelectric materials are available which demonstrate minimal dielectric constant variations as a result of these factors. However, such materials have the undesirable properties of low phase change with voltage which is the main parameter of interest.
For these reasons, a phase shifter capable of realtime calibration is desired to relax the need for such high material specifications during the fabrication process. Likewise, a ferroelectric phase shifter capable of performing a self-test to indicate valid phase shifter operation is desired.